1. Technical Field
This invention relates generally to electronic measuring and monitoring circuits and techniques, and more particularly to a method and circuit for producing an analog output signal having a magnitude proportional to the root-mean-square (RMS) value of an analog input signal.
2. Description of Related Art
The RMS value of a time-varying analog voltage indicates the equivalent direct current (DC) voltage in terms of power potential. For a pure sinusoidal waveform, the RMS value can be determined somewhat easily because it equals a known fixed percentage of the peak voltage. But more complex waveforms require more complex measuring techniques. This is important to the measurement industry, particularly the power measurement industry which bases charges for electricity upon RMS values. So, details of the techniques and related circuitry employed to determine the RMS value demand attention.
Perhaps the oldest method for measuring the RMS value of a voltage proceeds by applying the voltage to a filament or semiconductor device. As that is done, the rise in temperature of the device is measured as a direct indication of equivalent power (i.e., heating effect) of the applied voltage. But the need for a thermally insulated chamber and very delicate temperature measuring equipment make this method somewhat undesirable.
Another method for measuring the RMS value proceeds with analog circuitry by employing the logarithmic current-to-voltage characteristic of a forward-biased semiconductor junction. This method proceeds to calculate the RMS value by (i) converting the voltage to a current, (ii) passing the current through the forward-biased semiconductor junction, (iii) doubling the output voltage (equivalent to squaring the current), (iv) integrating the doubled voltage (averaging the squared current), and then (v) taking the antilog of the averaged squared current by applying it to a second forward-biased semiconductor junction opposite to that described above. Accuracy is dependent on how well the semiconductor junctions follow actual logarithmic curves.
The above makes it evident that existing methods for measuring RMS have certain drawbacks that need to be overcome. Thus, the measurement industry needs an improved method and circuit for measuring RMS values, especially of complex waveforms.